The statements in this section merely provide background information related to the present disclosure and may not constitute prior art.
High global oil prices, fuel efficiency due to CO2 regulations, and eco-environment concerns are important considerations in vehicle development. In order to achieve these goals, Automobile manufacturers are devoting their efforts into developing technologies to improve fuel efficiency.
It is important to reduce power loss of an automatic transmission to improve the fuel efficiency of a vehicle. In particular, it is important to reduce power loss of a torque converter.
Power loss of the torque converter always occurs since the torque converter of the automatic transmission transfers the power using oil, and a lock-up clutch system is used to supplement this weakness and to improve the efficiency. Hence, it has been developed in direction to expand operating area of the lock-up clutch to improve efficiency of the entire driving of vehicle. A typical example of the lock-up clutch is a 3-way separate chamber multi-plate clutch.
However, since the operating area of the lock-up clutch is expanded, a precise slip control is desired. In particular, we have discovered that an excellent gear change feel of the vehicle is desired.
Meanwhile, the gear change operation of the automatic transmission is performed by selectively engaging or releasing friction elements, the friction elements including a plurality of clutches and a brake.
For example, when three clutches are provided as friction factors, a desired gear change may be achieved by simultaneously releasing the second clutch and engaging the third clutch, while maintaining the engaged first clutch. Herein, the lock-up clutch absorbs gear change impact transferred to the vehicle by performing precise slip control and maintaining the pressure at a low level.
However, we have discovered that in the conventional art, when the control pressure of the lock-up clutch is only controlled to be maintained at a low level, some problems occur as described below.
First, an amount of oil in a pump is consumed equal to the volume of the third clutch, during an initial fill time period when the third clutch is operated and then the amount of oil in the lock-up clutch is reduced momentarily. Thus, pressure reduction occurs in the lock-up clutch, which causes impact to the vehicle.
Second, when the gear change operation is ended according to the engagement and release of the clutches, pressure of the third clutch is raised up to a line pressure. In this case, like the first problem, the amount of oil in the lock-up clutch is reduced equal to the amount of oil flowing into the third clutch. Thus, the pressure of the lock-up clutch is reduced, which also causes impact to the vehicle.
The foregoing is intended merely to aid in the understanding of the background of the present disclosure, and is not intended to mean that the present disclosure falls within the purview of the related art that is already known to those skilled in the art.